As a typical example of a conventional liquid jetting apparatus, there is an ink jet recording apparatus including an ink jet recording head for image recording. As other liquid jetting apparatuses, there are enumerated, for example, an apparatus including a color material jetting head used for color filter manufacture of a liquid crystal display or the like, an apparatus including an electrode material (conductive paste) jetting head used for electrode formation of an organic EL display, a surface emission display (FED) or the like, an apparatus including a living organic material jetting head used for biochip manufacture, an apparatus including a sample jetting head as a precision pipette, and the like.
In the ink jet recording apparatus as the typical example of the liquid jetting apparatus, an ink jet recording head including a pressure generation unit to pressurize a pressure generation chamber and a nozzle opening to jet pressurized ink as an ink droplet is mounted to a carriage.
In the ink jet recording apparatus, ink in an ink container continues to be supplied to the recording head through a flow path, so that printing can be continued. The ink container is constructed as, for example, a detachable cartridge which a user can easily exchange at a time point when the ink is used up.
Conventionally, as a management method of ink consumption of an ink cartridge, there is a method in which the number of jets of ink droplets in the recording head and the amount of ink sucked during the maintenance are accumulated by software, and the ink consumption is managed by calculation, or a method in which an electrode for detecting a liquid surface is attached to an ink cartridge, and a time point when a predetermined amount of ink is actually consumed is managed.
However, in the method in which the number of discharges of ink droplets and the amount of ink are accumulated by the software and the ink consumption is managed by calculation, there is a problem as described below. Variations in the weight of discharge ink droplets exist between heads. Although the variations in the weight of ink droplets do not have an influence on the picture quality, in view of a case where an error in the amount of ink consumption due to the variations is accumulated, ink whose amount includes a margin is filled in the ink cartridge.
Accordingly, there arises a problem that the ink corresponding to the margin remains according to an individual. On the other hand, in the method in which the time point when the ink is used up is managed by the electrode, since the actual amount of ink can be detected, the residual amount of ink can be managed at high reliability. However, since the detection of the liquid surface of ink depends on the conductivity of the ink, there are defects that the kind of detectable ink is limited, and the seal structure of the electrode becomes complicated. Besides, since a noble metal having superior conductivity and high corrosion resistance is generally used as a material of the electrode, the manufacturing cost of the ink cartridge runs up. Further, since two electrodes are required to be mounted, the manufacturing steps are increased, and as a result, the manufacturing cost runs up.
A device developed to solve the above problems is disclosed as a piezoelectric device in JP-A-2001-146024. This piezoelectric device can accurately detect the residual amount of liquid, eliminates the need for a complicated seal structure, and can be used while being mounted to a liquid container.
That is, according to the piezoelectric device disclosed in JP-A-2001-146024, by using that the resonant frequency of a residual vibration signal generated by the residual vibration (free vibration) of a vibration portion of the piezoelectric device after it is forcibly vibrated by a drive pulse is changed between a case where ink exists in a space opposite to the vibration portion of the piezoelectric device and a case where the ink does not exist, the residual amount of ink in the ink cartridge can be monitored.
FIG. 9 shows an actuator constituting the foregoing conventional piezoelectric device. This actuator 106 includes a substrate 178 having a circular opening 161 almost at the center, a vibration plate 176 disposed on one surface (hereinafter referred to as a “front surface”) of a substrate 178 so as to cover the opening 161, a piezoelectric layer 160 disposed at the side of the front surface of the vibration plate 176, an upper electrode 164 and a lower electrode 166 between which the piezoelectric layer 160 is sandwiched from both sides, an upper electrode terminal 168 electrically connected to the upper electrode 164, a lower electrode terminal 170 electrically connected to the lower electrode 166, and an auxiliary electrode 172 disposed between the upper electrode 164 and the upper electrode terminal 168 and electrically connecting both.
Each of the piezoelectric layer 160, the upper electrode 164, and the lower electrode 166 has a circular portion as a body portion. The respective circular portions of the piezoelectric layer 160, the upper electrode 164 and the lower electrode 166 form a piezoelectric element.
The vibration plate 176 is formed on the front surface of the substrate 178 so as to cover the opening 161. An actually vibrating vibration region in the vibration plate 176 is determined by the opening 161. A cavity 162 is formed of a portion of the vibration plate 176 facing the opening 161 and the opening 161 of the substrate (cavity formation member) 178. A surface (hereinafter referred to as a “back surface”) of the substrate 178 at the opposite side to the piezoelectric element faces the inside of an ink container. By this, the cavity 162 is constructed to come in contact with liquid (ink). Incidentally, the vibration plate 176 is liquid-tightly attached to the substrate 178, so that even if the liquid enters the cavity 162, the liquid does not leak to the front surface side of the substrate 178.
In the foregoing actuator 106 of the related art, residual vibration (free vibration) of the vibration portion generated after the vibration portion is forcibly vibrated by applying a drive pulse to the piezoelectric element is detected as counter electromotive force by the same piezoelectric element. Then, by using that the residual vibration state of the vibration part is changed in the vicinity of the time when the liquid surface in the ink container passes the setting position of the actuator 106 (strictly, the position of the cavity 162), the residual amount of ink in the ink container can be detected.
The foregoing conventional actuator (piezoelectric device) 106 is mounted to a container wall of a container body 181 of an ink cartridge 180 as shown in FIG. 10, and the cavity 162 to receive ink as a detection object is exposed in the ink reservoir space of the inside of the ink container 180.
However, as stated above, since the foregoing conventional actuator (piezoelectric device) 106 is constructed so that the cavity 162 is exposed in the ink reservoir space of the inside of the ink cartridge 180, when the ink in the inside of the ink cartridge 180 foams by the vibration or the like, an air bubble easily enters the cavity 162 of the actuator 106. When the air bubble enters the cavity 162 as stated above and stays there, the resonant frequency of the residual vibration detected by the actuator 106 becomes high although the residual amount of ink in the ink cartridge 180 is sufficient, and there is a problem that an erroneous judgment is made such that the liquid surface passes the position of the actuator 106 and the residual amount of ink has become small.
Besides, when the size of the cavity 162 of the actuator 106 is made small in order to detect the passing timing of the liquid surface with high accuracy, the meniscus of ink is apt to be formed in the cavity 162. Thus, even if the liquid surface passes the position of the cavity 162 by the consumption of ink, since ink remains in the inside of the cavity 162, there is a problem that an erroneous judgment is made such that the liquid surface does not pass the position of the actuator 106, and the residual amount of ink is sufficient.
As shown in FIGS. 6 to 8 in JP-A-2001-146024, this publication discloses that the length of the cavity in an electrode drawing-out direction in which the upper and lower electrodes are drawn out is larger than the length of the cavity in a direction perpendicular to the electrode drawing-out direction. In the case in which the planar shape of a cavity is long in one direction, an unnecessary vibration which is different from a residual vibration to be detected is included in the residual vibration (free vibration) generated in a vibration portion after a driving pulse is applied to a piezoelectric unit. As a result, there is a problem in that it is hard to reliably decide the presence of an ink.
It can be supposed that such an unnecessary vibration is generated because a difference in a vibration mode is great between a forcible vibration generated in the vibration portion when the driving pulse is applied to the piezoelectric unit and the residual vibration (free vibration) generated in the vibration portion after the forcible vibration.